PHOTOTROPISM 471 



the pigment that absorbs the light. Bandurski d al. (1950) have shown 

 that albino corn seedlings, from which they were unable to extract meas- 

 urable amounts of carotene, were as phototropically sensitive as ordinary 

 corn seedlings that did contain carotene. These same experiments had 

 been carried out by the present author in 1940 (unpublished). At that 

 time it was found that not a single one of the white corn coleoptiles was 

 without carotene. To demonstrate the presence of very minute amounts, 

 the petroleum-ether extract of individual coleoptiles was sucked through 

 a miniature chromatographic column made from a capillary tube filled 

 with alumina, and in every case the yellow carotene band appeared. It 

 was visible because the surface of the column was large compared with its 

 diameter. It therefore seems doubtful whether real carotene-free cole- 

 optile tips exist. 



3. The auxin extractable from the lower parts of the coleoptile is almost 

 certainly partially indoleacetic acid. The fact that growth may cease 

 almost completely while considerable amounts of indoleacetic acid remain 

 in the cells makes it likely that this generally distributed indoleacetic 

 acid is responsible for at most only a small fraction of the growth of the 

 coleoptile. 



4. As Galston and Baker (1949) have shown, the indoleacetic acid 

 is readily destroyed by light absorbed by riboflavin; this destruction 

 may account for the relatively small curvatures produced by basal 

 illumination. 



There is a different explanation possible for the effects of light on the 

 growth of the basal zones. Van Overbeek (1936) has shown that, when 

 Avena coleoptiles are preilluminated, their subsecjuent response to indole- 

 acetic acid is increased. This is obviously an indirect effect and may 

 be due to a change in the sensitivity of the cells to auxin. Therefore, 

 although the hypothesis of indoleacetic acid destruction on basal illumi- 

 nation of the coleoptile is very attractive, it is not necessarily the only 

 possible explanation. Since it appears that such a small fraction of the 

 extractable indoleacetic acid is destroyed, it would be very difficult to 

 decide whether or not the destroyed fraction had any direct relation to 

 growth. 



Several investigations have been made to estimate the absorption of 

 light within the illuminated coleoptile. The work of Nuernbergk (1927) 

 is the most extensive. He concluded that the decrease in hght intensity 

 from front to back is rather uniform in the basal zones of the coleoptile. 

 Here the enclosed primary leaf accounts for most of the light absorption. 

 The light intensity on the rear is about 3 per cent of the incident light. 

 In the extreme tip the light conditions were found to be more variable ; 

 no constant drop in light intensity was found. 



For many years there was considerable discussion of whether the 

 phototropic curvature was caused by the direction of hght rays or by 



